1. Field of the Invention
The present invention relates generally to methods and apparatus for illuminating flat panel displays. More particularly, the invention relates to methods and apparatus for providing substantially continuously-variable levels of illumination of such displays from very low levels to very high levels of display illumination.
2. Description of the Related Art
Flat panel displays (FPDs), such for example as those based on liquid crystal technologies, are typically illuminated using an electrically-activated source of light that is located at or proximate the back or rear side or face or peripheral edges of the display. In a common implementation of such FPD backlighting, an S-shaped or otherwise generally serpentine fluorescent tube is positioned behind the display to provide a substantially uniform illumination of the FPD screen when an electric current is applied to the fluorescent tube. It is also known to utilize a plurality of concurrently-activated fluorescent tubes to backlight a FPD, and although the present invention is for convenience described herein using a single-tube implementation, it should be understood that the invention is equally applicable to a multiple tube arrangement and, indeed, will typically be practiced using multiple tubes.
The increasing use of FPDs in aircraft—and most particularly in the cockpits or flight decks of modern commercial airliners to display to the pilot and flight crew a wide range of aircraft, flight, navigation and other data used in the operation and control of the aircraft—requires that the FPDs be illuminated in a manner that insures that pilots can quickly and easily view and locate the relevant information displayed thereon under a variety of ambient lighting conditions. In normal daylight conditions, for example, it may be necessary or appropriate to illuminate an FPD to a typical brightness level of somewhere between about 100 and 150 Foot-Lamberts (ft-L). Under night conditions, on the other hand, that same amount of illumination renders the FPD screen far too bright for use and could interfere with a pilot's ability to readily view and perceive other, less luminous objects both within the cockpit and outside of the aircraft; rather, at night a backlit FPD illumination of as little as one-thousandth ( 1/1000) to one-ten-thousandth ( 1/10,000) of the normal daytime illumination will normally suffice.
In operation, a fluorescent tube emits light when an electric current flowing through the tube ionizes a vapor confined within the tube to create a plasma or ion cloud which, in turn, causes a phosphor coating on the interior surface of the tube to fluoresce and thereby emit visible light. Thus, when an electrical potential is applied across the terminals of a fluorescent tube, the resulting current flow between the tube electrodes creates a plasma within the tube and the tube emits visible light. When application of the electrical potential across the terminals is discontinued or interrupted, the plasma dissipates and the tube ceases to emit light. Reapplication of the electrical potential across the tube terminals causes the plasma to reignite or regenerate and the tube to once again emit visible light.
The magnitude of the illumination emitted by a fluorescent tube can be varied through selective control of the amount of electrical current that operatively flows through the tube. It is accordingly possible to reduce or dim the FPD image brightness from that desirably utilized in normal “daylight” conditions by selectively reducing the electric current supply to the tube from that used to produce the daylight illumination level. The tube illumination level can be reduced in this manner—i.e. through straightforward controlled reduction of the electric current flowing through the tube—to dim the tube by a factor of about 100 without loss or dissipation of the plasma, but with continued reduction beyond that point the electric current is typically insufficient to maintain the plasma. In order to further reduce the emitted illumination to extremely low levels of brightness (e.g. to levels below about 1 ft-L), therefore, it is necessary to pulse width modulate the applied power and suitably adjust the duty cycle to obtain the desired further reduced illumination brightness levels.
The use of pulse width modulation to drive the fluorescent tube means that, in effect, the tube is being repeatedly switched “on” and “off” at a predetermined rate defined by the duty cycle. As the current flow ceases in the “off” period of each cycle, the plasma dissipates and must then be regenerated or reignited in the following “on” period of that (or of the next) cycle. Each time that the tube is switched or pulsed back “on”, a large rapid surge of current flows through the tube and there is a high energy burst as the plasma is regenerated, resulting in rapid wear on the tube cathode. Over time, these rapid and repeated restarts of a fluorescent tube have been found to reduce the effective life of the tube by as much as a factor of 10.